1. Field of the Invention
The present invention relates to a focus state detecting device utilizing image sensor arrays, and more particularly to an automatic focus state detecting device for use in automatic focusing control of a camera.
2. Related Background Art
There is already known a focus state detecting device of this kind, in which an image of the object formed by an objective lens (image taking lens) is focused again by a re-imaging optical system as two images onto a pair of image sensor arrays, and the focus state is calculated from the outputs of said image sensor arrays to determine the positional aberration of said two images, thereby detecting the focus state of the objective lens.
However, in a camera equipped with such focus state detecting device, if plural objects of different distances are present in a distance measuring zone defined in the view finder frame, there are often encountered inconveniences that an intermediate distance of such objects is identified as the best focus position, or that the focus state detection becomes impossible.
For this reason there is already known a method of dividing the distance measuring zone into plural blocks, and focusing the objective lens based on the results of plural focus state detections conducted in said plural blocks.
For example, the Japanese Laid-open Patents Sho 60-262004, Sho 61-55618, Sho 62-163007 and Sho 62-155608 disclose a method of dividing the image of the object by dividing the paired image sensor arrays into plural blocks, effecting the focus state detection in each of said plural blocks, selecting a block containing the closest object or the object with a largest contrast based on the results of said plural focus state detections, and focusing the objective lens according to the result of focus state detection in thus selected block.
Said paired image sensor arrays are generally composed of CCD image sensors.
Now reference is made to FIG. 1 for explaining the function of a CCD image sensor.
FIG. 1A shows the structure of a CCD image sensor, which is composed of a photosensor array 10 consisting of plural photosensors for photoelectric conversion, a shift register 40 for charge transfer, and a shift gate 30 for transferring the charges generated and accumulated in the photosensors of the array 10 to the shift register 40.
The shift register 40, receiving a two-phase clock signal consisting of a clock signal CLK1 and a phase-inverted clock signal CLK2, is connected to an output terminal OUT through a voltage conversion unit 41 and a buffer 42.
The photosensor array 10 receives a clear pulse CLR for controlling the accumulating operation from a terminal CLR. The shift gate 30 positioned between the photosensor array 10 and the shift register 40 is connected to a terminal SH.
The function of such CCD image sensor will be explained in the following, with reference to FIG. 1B.
At a time T1, the terminal CLR is shifted from the H-level to the L-level, whereby the charges generated in the photosensor array 10 start to be accumulated in respective photosensors. At a time T2 after the lapse of a predetermined time, an L-level pulse is given to the terminal SH whereby the charges generated in the photosensors of the array 10 in a period from T1 to T2 are transferred in parallel manner to the shift register 40. The shift register 40 transfers said charges from left to right by the two-phase clock signals consisting of the clock signal CLK1 and the phase-inverted clock signal thereof. Said charges are converted into voltages by the voltage conversion unit 41, and are time-sequentially released through the buffer 42 and the terminal OUT.
The predetermined time from T1 to T2 is called the accumulation time, which is selected shorter or longer as the brightness of the object is higher or lower, in order to obtain a contrast enabling the focus state detecting operation, thereby obtaining an almost constant output regardless of the brightness of the object.
However such prior art has been associated with the following drawbacks in case plural objects of different brightnesses are present in the distance measuring zone.
FIG. 2A shows an example of the object field in the view finder, containing a distance measuring zone 50. The image of the object present in said zone 50 is projected onto the image sensor arrays, and said zone 50 is divided into three blocks 50a, 50b and 50c.
Now let us consider an example of an object A present in the block 50a and a more distant object B in the block 50b. It is assumed that the background is sky with highest brightness, and that the brightness becomes lower in the order of the object B and then the object A. When such objects A, B and the background sky are projected on the image sensors, the selection of the accumulating time is difficult to determine.
If the accumulation time is determined based on the brightness of the object B, the image sensor array will provide an output as shown in FIG. 2B, wherein the output of the block 50a is too small for the focus state detecting operation. While the block 50b provides an output suitable for the focus state detecting operation, the block 50c, only containing the sky, provides an output but the focus state detecting operation is impossible because of lack of contrast. In this case, therefore, the focus state detecting operation is possible only for the object B present in the block 50b, and the objective lens is so driven as to be focused to said object B.
Then, if the accumulation time is determined longer than in the case of FIG. 2B, based on the brightness of the object A, the image sensor array provides an output as shown in FIG. 2C, wherein the block 50a provides an output with a contrast suitable for the focus state detecting operation, but the blocks 50b and 50c provide saturated output without contrast because of the excessively high brightness in comparison with the accumulation time. Consequently the objective lens is so driven as to be focused to the object A present in the block 50a.
On the other hand, if the accumulation time is determined according to the brightness of the background sky, there is obtained an output as shown in FIG. 2D, wherein the focus state detecting operation is impossible in the block 50a because of the low output. Said operation is possible in the block 50b but is unstable because of the low output, and is impossible in the block 50c because of lack of contrast. Thus the objective lens is so driven as to be focused to the object B in the block 50b, but precise focusing cannot be achieved because the low output leads to an unstable result of focus state detection.
Thus, if plural objects with significant difference in brightness are present in the distance measuring zone, the object to be focused may be undesirably selected or the precision of focusing may be degraded, depending on the method of selecting the accumulation time for the image sensor arrays. For example, when it is desired to focus the lens to the closest object in the distance measuring zone, the focus state detecting operation for such object becomes impossible if an accumulation time suitable for another object is selected.